Directional coupler



Sept. 25, 1956 o. o. FIET DIRECTIONAL coUPLER Filed Feb. l, 1952 INVENTOR owe@ QF e Mffmm United States Patent O 'nmncrIoNAL coUrLEn Owen Orlando Fiet, Oaklyn, N. J., assignor to RadioCorporation of America, a corporation of Delaware Application February 1, 1952, Serial No. 269,542

12 Claims. (Cl. S33-10) This invention relates to transmission line coupling arrangements and particularly to a directional coupler.

it is known in prior art arrangements to utilize a directionally sensitive coupling loop within ya transmission line of the type having 'an enclosing metallic side Wall, such as coaxial lines and waveguides. A directional coupling loop of this type usually has a terminating resistor connected between one end of the loop and the side wall of a main transmission line. The other 4end of the coupling loop is connected to one conductor of a branch or output transmission line. The other conductor of the branch or output transmission line is directly connected to the side wall or outer conductor of the main transmission line.

ln certain applications, it is an important requirement that any energy travelling in the branch transmission line toward the coupling loop not be reflected by the coupling loop and terminating resistor itself. Considered from the branch output transmission line, the coupler must appear as a perfect generator having a dissipative internal irnpedance the same as that of the branch output transmission line to which it is connected. For example, in cases where a balanced mixer is connected to the branch line, stray energy from the local oscillator may be propagated down the branch line to the coupler. Reflections of this stray energy may produce false and undesired mixer product currents in the output of the mixer.

Directional couplers may utilize slots, probes, or loops. The directional coupler of the present invention is primarily concerned with improving the characteristics of loop-type couplers so that they'sare utilizable over a very wide frequency range and provide a nonreecting termination for the line into which the sample of energy is to be coupled.

Accordingly, it is an object of this invention to provide an improved directional coupler of the loop type.

Another object of this invention is to provide a directional coupler which has a high degree of directional sensitivity and which appears as a perfect generator having a dissipative internal impedance the same as that of the branch output transmission line to which it is connected.

Another object of this invention is to provide an improved directional coupler which greatly reduces reecy tions of energy propagated toward the coupler in the branch output transmission line to which it is directly connected.

A further object of this invention is to provide a coupler of the loop type with improved directional sensitivity operable over a wide frequency range of the order of 20 to l.

Brieliy, in `accordance with the present invention there is provided a directional coupler of the loop type which couples a portion of the energy propagated in one direction in a main transmission line into a branch transmission line. Means are provided in the coupler to 'adjust and preferably to equalize therelative magnitudes of the currents in the coupling loop which Iare induced by the electrostatic and electromagnetic fields within the main transmission line.

In a preferred embodiment, two loops are utilized.

Patented Sept. 25, 1956 One loop is connected at one end to the branch transmission line, as for example the inner conductor of a coaxial cable, and at the other end is connected to a terminating resistor, in turn, connected to the outer conductor of the main transmission line. The other loop forms an electrostatic shield land may be a simple wire which, together with the supporting body of the directional coupler, transversely encircles the first loop. The first loop is proportioned in such manner and is spaced from the body ofthe directional coupler so that the characteristic impedance between the first loop and the body of the coupler i's the same as the impedance of the branch transmission line intowhich the signals 'are to be coupled. The terminating resistor for the first loop has a value of resistance which is the same as this characteristic impedance.

A more detailed description follows in conjunction with the accompanying drawing in which:

Fig. l shows a view in cross section of the dierctional coupler of the present invention positioned to extract energy from a main coaxial transmission line;

Fig. 2 is an end view of the structure shown in Fig. l;

Fig. 3v is a view in section along the line III of Fig. 1 with the coupler turned 'at an angle to the axis of the main transmission line;

Fig. 4 `shows a view, in cross section, of a portion of a modiiication of a directional coupler of the invention; and

Fig. 5 shows a view, in cross section, of a portion of another modification of the directional coupler of the invention.

Referring to Fig. l, there is shown a main coaxial transmission line having an inner conductor 11 land an outer conductor 13. This main line l1, 13 is illustrative of any suitable current-carrying line having :an enclosing metallic side wall, such as a waveguide. The directional coupler of this invention is carried by a housing 15 and includes a metallic body portion 17, lan output coaxial transmission line having an inner conductor 19 and :an outer conductor 21, a metallic strip 23 having one end connected to the inner conductor 19 of the branch output transmission line and its other end connected to a terminating resistor 25 which is seated within a terminal block 27 and an electrostatic shielding loop 29. The housing 15 is mechanically attached and electrically connected to the outer conductor 13 of the main transmission line. rl`he coupler body 17 is electrically connected and mechanically -secured within a hole in the housing 15 and is carried thereby. A circular aperture in the coupler body 17 receives the coaxial transmission line 19, 21. The transmission line section 19, 21 can be considered as a branch output transmission line to which part of the energy propagated in the main transmission line 11, 13 is to be coupled.

The lower end of the inner conductor 19 of the branch transmission line extend-sheyond the termination of' its outer conductor 21, and is connected to lthe flat metallic strip 23 whose long axis is in a plane parallel to the axis of the main transmission line lll, 13. The end of the terminating resistor 25 which is remote from the i-at strip 23 is electrically connected to the terminal block 27. The long axis of the at metallic strip 23 is preferably rotatable in its plane of position with respect to the axis of the main transmission line 1l, 13. This rotatability allows the hat strip 23 to be skewed with respect to the axis of the main transmission line 11, 13.

For very high and ultra high frequency applications, it is desirable lthat the terminating resistor 2S have negligible reactance. A resistor which is suitable for this pur-pose may be, for example, a pyrolytic carbon coating on a ceramic or glass rod, or a platinum gold iired coating on ceramic or glass. The absolute resistance of such resistors changes only very slightly with `frequency discussion.

and these types -of resistors appear non-reactive far up in the `ultra high frequency ran-ge.

The terminal block 27 is secured within a recess in the coupler body 17 and forms a part of the electrical kpath between the 'terminating resistor 25 and the outer conductor 13 of the `main transmission line 1-1, 13. The terminal block 27 preferably has a Vtapered recess therein -to receive the terminating resistor 25. The vtaper of such a recess theoretically should be exponential to minimize retiections from the terminating `resistor end of the cou- Ipling loop. However, for many applications, straight lconical taper to the sides of the recess in the terminal Vblock 27 operates practically as well.

The terminal-blo'cx 27 uses an exponential or conical 'taper to obtain good impedance characteristics for the dissipative terminating resistor y25 at higher frequencies. The length of the terminating resistor 25 and consequently the lconfiguration of terminal block 27 depends upon how much power must be dissipated in the terminating resistor A2S. lf larger output voltages are desired, higher wattage resistors must be employed. For example, a 5%; inch long, one watt, 5() ohm :resistor will permit the directional coupler to develop an output voltage of v7 volts R. M. S. valuc. For a l() volt R. M. S. output, a l1/2 linch long, 2 watt, 50 ohm resistor is required.

Tofmatch the coupler to its branch output transmission line 19, 21 so that it appears as a perfect generator having an internal impedance the same as that of the branch :transmission line y19, 21, the dimensions of the strip 23 :and `its spacing from the adjacent coupler body 17 are Aproportioned to have a characteristic impedance the same *nating vresistor should form a non-reflective termination yhaving the same impedance value as the branch output transmission line over the range of frequencies to be sampled. The shape of the terminal block 27, therefore,

is not limited to the precise forms of taper shown.

That the coupling arrangement has directional sensirivi'ty may be understood from the following qualitative The current induced by the electromagnetic field flows in one direction toward either one end or the other of the strip 23. The current due to the electrostatic eld lio-ws either away from or toward the electrical center of the strip 23. For a signal propagated in one direction inthe main transmission line 11, 13, there lis a given conguration and phase relation of the electrovstatic and electromagnetic fields. For this given configuration `and phase relationship, the currents induced in thc loop 1.9, 23, 2S are in opposition on the side toward the terminating resistor 25 and are therefore balanced out. AOn the side of the loop toward the inner conductor l19 of the branch output transmission line, these currents additively combine.

For propagation in the opposite direction in the main transmission line 11, 13, the electromagnetic field vector existing in the vicinity of the loop 19, 23, 25 and the electrostatic field vector are in opposite phase with respect to each other from their relative phase in the condition just described. The currents induced bythe electromagnetic and electrostatic fields of this second case cancel in the inner conductor 19 of the branch output transmission line and-add in the terminating resistor `25. lf the strip 23 is perfectly matched, from an impedance standpoint, 4to the terminating resistor 25 on one side and to the branch output transmission line 19, 21 on the other side, the currents in the coupling loop 19, 23, 25 resulting from propagation in the main transmission line 11, 13 in one direction are dissipated in the terminating resistor 25. Currents resulting from propagation yin the main transmission 'line 11, 13 in the other direction are coupled into the branch output transmission line 19, 21.

it has been assumed in the `preceding discussion that 'the currents 'in the `coupling loop 19, 23, 25 :induced by 4 the electromagnetic field and those induced by the electrostatic field have the same magnitude. In the coupler of this invention, the proportions of the strip 23 and its spacing from the adjacent coupler body 17 are arranged to give a characteristic impedance the same as that of the branch transmission line 19, 21 (for example, in the order of 50 to 90 ohms). When so proportioned, the coupler has excellent impedance versus frequency characteristics and appears as a perfect generator to the branch output transmission line 19, 21 over a very wide range of frequencies, which is highly advantageous. When the impedance requirement is fulfilled by so proportioning the elements, the electrostatic coupling is increased due to increase in the area of the strip 23 which is subject to the electrostatic field, without a corresponding increase in magnetic coupling. Therefore, the currents induced in the coupling loop 19, 23, 25 by the electrostatic field, regardless of the direction of propagation, are considerably. in excess of those induced by the electromagnetic field. Consequently, the directional sensitivity of the coupler is somewhat impaired because of .this excess electrostatic coupling.

This defect of electrostatic coupling is overcome, and the excellent impedance versus frequency characteristics retained, by providing a suitable partial electrostatic shielding means. Such .a shielding arrangement preferably .furnishes just enough screening to cause the currents in the strip 23 induced respectively by the electrostatic and electromagnetic fields to be equal in amplitude. This electrostatic shielding is accomplished, for example, by a second metallic loop 29 which encircles 4the strip 23 transverse to the axis thereof and is spaced from the strip 23. This shielding loop 29 is connected at both ends tothe coupler body 17, and in its simplest form is merely a turn of wire.

If the area of the strip 23 which couples with the electrostatic field is' considerably increased to meet the impedance condition between the strip 23 and the coupler body 17, additional shielding loops may be included to provide sufficient screening to bring the effective currents induced in the strip 23 by the electnostatic and electromagnetic elds respectively to the same value. Alternatively, the same result may be accomplished by making lthe shielding loop 29 with a larger dimension in the direction of the length of the strip 23. In such an alternative arrangement, the shielding loop 29 would appear as a U-shaped flat strip loop.

As .mentioned above, the axis of the coupling loop 19, 23, 25 is preferably adjustable so that it may be skewed with respect to the axis of the main transmission line 11, 13. This adjustability permits the electromagnetic field currents to be increased or decreased to give a very fine balance between Vthe :currents induced by the electrostatic and electromagnetic fields in the coupling loop 19, 23, 25. For example, let us assume 'that the electrostatic shielding furnished by the shielding loop 29 is calculated to allow the currents induced in the flat strip 23 lby the magnetic field 4to exceed by 5 percent those induced by the electrostatic field for maximum electromagnetic coupling (that is, with the -axis ofthe strip 23 parallel to the axis 'of the main transmission yline 11, 13). If now, the coupler body 17 Iis rotated vin the housing 15 about eighteen degrees, the effective magnetic coupling is reduced to 0.95 of its maximum value, since the degree of magnetic coupling varies vas the cosine of the angle between the Vnot 'be .increased lrelative to the electrostatic field coupling by rotating the coupler body 17 since the electromagnetic coupling is already set at maximum.

Referring to Fig. 2, which shows an end view of the coupler of Fig. 1, the coniiguration of the shielding loop 29 and its position relative to the strip 23 is readily apparent. The plane of the shielding loop 29 is shown as being transverse to the long axis of the strip 23 and therefore transverse to the axis of the main transmission line 11, 13 from which the sample of energyis to be extracted.

In Fig. 3 there is shown a' sectional view taken along the line III of Fig. 1, modified to the extent that the long axis of the flat strip 23 is shown ashaving an acuteA skew angle A with respect to the axis of the main transmission line 11, 13. The shielding loop 29 is shown rotated with the flat strip 23 so that it lies in a plane transverse to the long axis of the strip 23, but is no longer transverse to the axis of the main transmission line 11, 13, as described in connection with Fig. 2.

Referring to Fig. 4, there is illustrated an alternative shielding loop 31 having a larger dimension along the long axis of the flat strip Z3 than that shown and described with respect to Figs; l, 2 and 3 above. The arrangement shown in this figure is particularly useful where a change in the dimensions of the fiat strip 23 has greatly increased the excess of electrostatic coupling and requires a more complete shielding loop to equalize the currents. induced in the coupling loop 19, 23, 25 by the electrostatic and electromagnetic fields.

Fig. 5 shows another alternative shielding arrangement which utilizes a grid-like shield 33. Such a grid-like shield 33 may be substituted for a plurality of individual shielding loops 29 discussed above in connection with Fig. 1, and is the electrical equivalent thereof.

Also shown in Fig. 5 is an alternative terminal block 27 proportioned to accommodate a terminating resistor 25 with a length greater than the terminating resistor 25 shown in Fig. 1. The terminal block 27' has a taper so that the spacing between the inside surface of the terminal block 27 and the terminating resistor 25' varies exponentially, as shown. In this arrangement, as with that described above in connection with Fig. 1, the terminal block 27' in combination with the terminating resistor 25 forms a non-reflective dissipative termination having the same impedance value as the output branch transmission line 19, 21.

One embodiment of the invention successfully tried out in practice was operable over a range of frequencies from 50 megacycles to more than 1,000 megacycles and presented a source impedance of 50 ohms. The conductor 13 of the main transmission line had an outside diameter of 3% and was cylindrical copper tubing. The housing 15 was brass with a cylindrical surface of the same curvature as the outer conductor 13 and contacted the outer conductor 13 of the main transmission line. The circular aperture in the housing 15 receiving the coupler body 17 had a diameter of 11/2". The coupler body 17 was a brass cylinder 11/2" in diameter and 3 long. The output transmission line 19, 21 had a characteristic impedance of 50 ohms and an outside diameter of 53", fitting in a 5% circular hole in the coupler body 17. Another hole in the coupler body 17, to receive the terminating block 27, was 3A in diameter. The flat strip 23 was of brass and 1%" long by wide and had rounded ends. The inner conductor 19 of the branch output transmission line was secured within a hole at one endof the strip23, and the terminating resistor 25, a 50 ohm pyrolytic carbon resistor coating on a %2 ceramic rod, was secured within a hole at the other end ofthe strip 23. The shielding loop 29 was No. 12 gauge copper wire secured at both its ends to the coupler body 17. The shielding loop 29 was spaced from the at strip 23 approximately W16 and was positioned in a plane near the middle of the length of the fiat strip 23.

The coupler body 17 was made adjustable in and out of -the housing 15, and rotatable in the housing aperture. A locking arrangement allowed the coupler body to be set up to any desired depth of insertion or angle of rotation within the housing 15. By virtue of 'this arrangement, the desired direction of sampling of signals within the main transmission line 11, 13' may be reversed by turning the coupler body 17 around in the housing 15.

Although the coupler of this invention has been shown and described in connection with sampling signals from a main transmission line 11, 13 of the coaxial type, it is to be understood that the invention is operative also with either cylindrical or rectangular waveguides and other types of transmission lines having an enclosing metallic side Wall.

I claim:

1. In combination, a first transmission line having an enclosing metallic side wall and adapted to carry electrical energy, a branch transmission line having inner and outer conductors, the branch line outer conductor being connected to the side wall of the rst line, a metallic coupling loop having one end connected through a resistance to said side wall and connected at the other end to said inner conductor, and a second metallic shielding loop within said first line having its ends connected to said side wall, saidisecond loop together with said side wall surrounding said rst loop, whereby said second loop electrostatically shields the iirst.

2. A directional coupler for sampling energy in av first transmission line having an enclosing metallic side wall, comprising a branch transmission line having inner and outer conductors, the outer conductor of said branch line being connected to the'vside wall of said first line, a metallic coupling loop having one end connected through a resistance to said side wall and connected at the other end to said inner conductor of said branch line, said resistance having an ohmic yvalue equal to the characteristic impedance of said branch transmission line, and a second metallic shielding loop within said iirst line having its ends connected to said side wall, said second loop together with said side wall surrounding said rst loop, whereby said second loop electrostatically shields the lrst.

3. A directional coupler for sampling energy in a first transmission line having an enclosing metallic side wail comprising a branch transmission line having inner and outer conductors, the outer conductors of said branch line being connected to the side wall of said irst line, a metallic coupling loop within said first line having one end connected through a resistance to said side wall and connected at the other end to said inner conductor of said Vbranch line, said resistance having an ohmic value equal loop within said first line to adjust the relative magnitude of the currents in said coupling loop respectively induced by the electrostatic and electromagnetic fields therein.

4. A directional coupler for sampling energy in a first transmission line having an enclosing metallic side wall comprising a branch transmission line having inner and outer conductors, the outer conductor of said branch line being connected to the side wall of said first line, a metallic coupling loop having one end connected through a resistance to said side wall and connected at the other end to said inner conductor of said branch line, said resistance having an ohmic value equal to the characteristic irnpedance of said branch transmission line, said coupling loop being spaced from said side wall structure to present an impedance between said loop and said side wall equal to said characteristic impedance of said branch transmission line, and electrostatic shielding means within said iirst line and around said coupling loop to adjustl the relative magnitude of the currents in said coupling loop respectively induced by the electrostatic and elec-- comprising a branch transmission line having inner and'.

outer conductors, the outer conductors of said branch line'being connected tothe'side Wall of said iirst line, a metallic coupling,y loop'havingone end connected through a-re`sistance 'tot said side wall and connected at the other end to' said inner conductor of said branch line,y said coupling loop being spaced from said side Wall structure to present an impedance between said loop and said side wall equal to saidv characteristic impedance of said branch transmission line, and4 a second metallic shielding loop within' saidl first line having itsy ends connected to said side wall, said second loopy together with said side wall surroundingsa'id irstloopgwhereby said second loop electrostatically shields the rst.

6. A directional coupler for sampling energy in a irst transmission line having an enclosing metallic side wall comprising a branch transmission line havingl inner and outer conductors, the outer conductor of said branch line being connected to 'the side wall of said first line, a metallic coupling-loop'havin'g one endconnected to said inner conductor of said branchline and the other end connected through a non-reflective resistance termination to said side wall, said resistance termination having an ohrnic value equal to the characteristic impedance of said branch transmissionline, and a second metallic shielding loop within said first' line having its ends connected to said side wall, said second loop together with said side wall surrounding said iirst loop, whereby said second loop electrostatically shields the first.

7. A directional coupler for use in a irst transmission line having an enclosing metallicside wall, comprising a branch'transmission line having inner andouter conductors`,rthe'outer conductor of said'branch line being conne'ctedto'the'side wall of' said'rst line, a metallic coupling-loop havingl one end connected to said inner conductor of said branch line, and a second metallic shielding loop exending transversely around said coupling loop and having its ends connected to said'metallic side wall.

8. A directional coupler dened in claim 7, and in addition, a resistor connected between the other end of said metallic coupling-loop and said metallic side wall.

9. A directional coupler as detined in claim 7, wherein said shielding loop is a U-shaped flat strip.

10. A directional coupler as dened in claim 7, wherein said shielding loop is a metallic grid.

11. A directional coupler as deiined in claim 7, wherein said loops are rotatable as a unit.

12. A coupler for use in a iirst transmission line having an enclosing metallic sidewall, comprising a branch transmissionline having inner and outer conductors, the outer conductor of said branch line being connected to the side- Wall of said first line, a metallic coupling loop having one end connected to said inner conductor of said branch line, and means in said first transmission line to partially shield said coupling loop from the electric eld in said first transmission line.

References Cited in the tile of this patent UNITED STATES PATENTS 2,423,416 Sontheimer et al. July 1, 1947' 2,438,915 Hansen Apr. 6, 1948 2,527,979 Woodward Oct. 31, 1950 2,575,571 Wheeler Nov. 20, 1951 2,576,979 Thompson Dec. 4, 1951 2,615,958 Phillips Oct. 28, 1952 2,615,982 Zaslavsky Oct. 28, 1952 2,636,082 Saad Apr. 21, 1953 FOREIGN PATENTS 604,987 Great Britain July 14, 1948 625,378 Great Britain June 27, 1949 

